In MBMS architecture, the BM-SC is the Broadcast Multicast Service Centre, which is the application level server providing the multimedia content. This is illustrated generally in FIG. 8.
The MBMS GW is responsible for the user plane processing of the MBMS data, including such functions as content synchronization and delivering the data over a multicast IP transport to the relevant eNodeBs. The MBMS GW also executes control over the start and stop of the services and acts as a mediator between the access agnostic multimedia content sources and the LTE specific access network.
The MCE (MBMS Control Entity) is a radio resource control entity, which is responsible mainly for the coordinated allocation of radio resources over multiple cells in case of Single Frequency Network (SFN) transmission mode.
To efficiently support the distribution of broadcast/multicast data in cellular systems (e.g., the distribution of multimedia content, TV channels), the concept of Single Frequency Network (SFN) transmission is often used. This means that the same content is sent from multiple base stations in a time synchronized manner, which allows the receiving user terminal to combine the signals from multiple base stations and thereby achieve a good reception quality also at the cell edge.
In order the SFN concept to work, a mechanism is needed that achieves the synchronization of both the content, meaning that the same data is sent from multiple base stations in the same radio resource block and also the time synchronization of the base stations, meaning that the transmission in the identical radio resource blocks at multiple base stations are time aligned accurately enough.
The SFN concept is used, for instance, for the realization of the Multimedia Broadcast Multicast Service (MBMS) in UTRAN and the same principle will be used in the LTE system as well.
In UTRAN the MBMS content synchronization is done by the RNC node as part of the resource and scheduling control functionality in the MAC layer. However, in order this solution to work it is required to locate the radio interface scheduling and L2 processing (e.g., segmentation) in a central user plane processing node. In LTE these radio resource control functions will be located in the eNodeB along with the corresponding MAC and lower layer protocol layers. Therefore the earlier solutions are not favorable for LTE and they are not directly applicable either, unless some of the L2 protocol layers (e.g., RLC/MAC) are moved from the eNodeB to the central MBMS Gateway (MBMS GW) node or some new protocol layers with L2 functionality (e.g., segmentation/concatenation) are introduced between the RLC/MAC and upper layers.
Similar solutions are being discussed also for LTE, where the user plane processing in the MBMS GW would include RLC/MAC layer or a newly introduced protocol layer doing the segmentation/concatenation according to the radio resource block size.
These solutions are not desirable in LTE since they would require RAN functionality in the MBMS GW node, which would add to the complexity of the node and would significantly differ from the user plane processing functions in the central node used for unicast traffic.
It would be desirable to use a content synchronization method that allows keeping the user plane processing in the MBMS GW node as simple as possible and free of any RAN specific processing, i.e., similar to the user plane processing functionality for unicast traffic.